Magnetization and magnetic susceptibility measured in the zero-field-cooled mode were used to study the spin
crossover transition in polycrystalline CuxCoyCrzSe4 compounds. With increasing Co content a transition from
ferromagnetic order via ferrimagnetic one to antiferromagnetic-like behaviour was observed. This transition is
accompanied with a lowering symmetry from cubic to monoclinic and for the latter the spin crossover phenomenon occurs. These results are considered in a framework of the ligand–field split and the spin–orbit coupling

Groń, Tadeusz

Krok-Kowalski, Józef

Maciążek, Ewa

Mydlarz, T.

Pacyna, A. W.

Zawisza, Beata

Acta Physica Polonica A

English

Repozytorium Uniwersytetu Śląskiego RE-BUŚ

             Title: Spin crossover in CuxCoyCrzSe4 semiconductors  Author: Ewa Maciążek, Tadeusz Groń, A.W. Pacyna, T. Mydlarz, Beata Zawisza, Józef Krok-Kowalski  Citation style: Maciążek Ewa, Groń Tadeusz, Pacyna A.W., Mydlarz T., Zawisza Beata, Krok-Kowalski Józef. (2011). Spin crossover in CuxCoyCrzSe4 semiconductors. "Acta Physica Polonica A" (Vol. 119, nr 5 (2011), s. 711-713).  Vol. 119 (2011) ACTA PHYSICA POLONICA A No. 5XXXIX “Jaszowiec” International School and Conference on the Physics of Semiconductors, Krynica-Zdrój 2010Spin Crossover in CuxCoyCrzSe4 SemiconductorsE. Maciążeka, T. Grońb, A.W. Pacynac, T. Mydlarzd, B. Zawiszaaand J. Krok-KowalskibaInstitute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, PolandbInstitute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, PolandcThe Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of SciencesE. Radzikowskiego 152, 31-342 Kraków, PolanddInternational Laboratory of High Magnetic Fields and Low TemperaturesGajowicka 95, 53-529 Wrocław, PolandMagnetization and magnetic susceptibility measured in the zero-field-cooled mode were used to study the spincrossover transition in polycrystalline CuxCoyCrzSe4 compounds. With increasing Co content a transition fromferromagnetic order via ferrimagnetic one to antiferromagnetic-like behaviour was observed. This transition isaccompanied with a lowering symmetry from cubic to monoclinic and for the latter the spin crossover phenomenonoccurs. These results are considered in a framework of the ligand–field split and the spin–orbit coupling.PACS: 75.30.Wx, 75.50.Dd, 75.50.Ee1. IntroductionStructural and electrical investigations carried out onthe polycrystalline Cu1−xCoxCr2Se4 compounds in thecompositional range 0.0 ≤ x ≤ 1.0 revealed semiconduct-ing properties and a transformation from cubic spinel--type to monoclinic Cr3S4-type structure above x =0.4 [1]. Recently, the X-ray diffraction studies on thepolycrystalline Cu1−xCoxCr2Se4 system (x = 0.0, 0.2,0.4, 0.6, 0.8, 1.0) proved the existence of the cubicspinel-type structure for x ≤ 0.2 and the monoclinicCr3S4-type one for x ≥ 0.8 [2]. CoCr2Se4 undergoesan order–disorder phase transition above 750 ◦C [2] andhas a sharp peak in its susceptibility at 199 K suggest-ing antiferromagnetic (AFM) behaviour [3]. Electricaland magnetic studies carried out on single-crystallinenon-stoichiometric CuxCoyCrzSe4 spinels showed ferro-magnetic (FM) ordering and p-type metallic conductiv-ity for y = 0.06, 0.1 and 0.11 as well as a ferrimag-netic behaviour and n-type electrical semiconductivityfor y = 0.23 [4].It is well known in literature that compounds con-taining transition metals with d4, d5, d6, and d7 elec-tron configurations usually are capable of forming eitherspin-free (high-spin, HS) or spin-paired (low-spin, LS)grounds states, depending on the strength of the cubicligand field, ∆, relative to the mean spin-pairing energy,P¯ [5]. Then, their magnetism follows the Curie–Weisslaw. However, when |∆ − P | becomes comparable tothermal energy, kT , both spin states HS and LS may bepopulated and cross over in thermal equilibrium. Thisphenomenon is commonly known as spin crossover forthat the Curie–Weiss law is not valid. So, the easiest wayof following the spin conversion as a function of tempera-ture is the measurement of the magnetic susceptibility [5].It seems thus natural to expect a spin transition inthe polycrystalline CuxCoyCrzSe4 system for higher Cocontent.2. Experimental detailsPowder samples of the CuxCoyCrzSe4 system were ob-tained as ceramics using a preparation method which isdescribed in detail elsewhere [1]. Chemical compositionof the compounds was determined by energy-dispersiveX-ray fluorescence (EDXRF) spectrometry. The sampleswere excited by the Rh target X-ray tube (XTF 5011/75, Oxford Instruments, USA). The X-ray spectra of thesamples were collected by thermoelectrically cooled Si--PIN detector (XR-100CR Amptek, Bedford, MA, USA)of 145 eV resolution at 5.9 keV. The EDXRF spectrom-eter is described in detail in Ref. [6]. The quantitativeEDXRF analysis of samples was performed by fundamen-tal parameters method based on the Sherman equation.The compositions studied in this paper are the same asreported in Ref. [1].Magnetization and magnetic susceptibility were mea-sured in the zero-field-cooled mode using a vibrating sam-ple magnetometer with a step motor [7] at 4.2 K and inapplied external magnetic fields up to 150 kOe and aFaraday type Cahn RG automatic electrobalance in thetemperature range 4.2–400 K and at 800 Oe, respectively.(711)712 E. Maciążek et al.3. Results and discussionFigure 1 shows the magnetization isotherms mea-sured at 4.2 K. The spinel samples with low Cocontent up to y = 0.25 reach full saturation of4.65 µB/f.u. for Cu0.93Co0.15Cr1.64Se4 and of 4.35 µB/f.u.for Cu0.75Co0.25Cr2Se4 at 140 kOe close to the theoret-ical value of 5 µB per molecule for Cu[Cr3+Cr4+]Se4spinel [8]. In these spinels the magnetic moments or-der ferromagnetically and the Curie temperature TC de-creases from 390 K for y = 0.15 to 377 K for y = 0.25(see Fig. 2). A pronounced decrease of saturation mo-ment with increasing Co content suggests the LS stateof the Co2+ ions, tetrahedrally coordinated, with spinS = 1/2 in the t2 orbital (see Fig. 3), because the Cr3+ions, octahedrally coordinated, with spin S = 3/2 in thet2g orbital are always in the HS state. Eventually, a FMand AFM coupling between Co2+ and Cr3+ magneticmoments may occur, too.Fig. 1. Magnetic moment µ vs. magnetic field H forthe CuxCoyCrzSe4 system at 4.2 K.The monoclinic Cr3S4-type structures with high Cocontent above y = 0.8 show a dramatically strong dropof magnetic moment up to 0.18 µB/f.u. at 137 kOe fory = 0.89. They do not reveal both saturation andspontaneous magnetization (Fig. 1). The temperaturedependence of magnetic susceptibility, χσ(T ), depictedin Fig. 4 reveals ferrimagnetic order with TC = 133 Kand without a Curie–Weiss behavior in the temperaturerange 4.2–400 K for Cu0.14Co0.87Cr1.83Se4. Above TC theχσ(T ) curve shows minimum at T1 = 168 K and maxi-mum at T2 = 255 K characteristic for a spin crossovertransition. The non-linear dependence of 1/χσ(T ) at800 Oe maybe a consequence of the strong dependence ofthe effective magnetic moment with temperature, whichis experimentally observed in the compounds where thespin crossover phenomenon occurs [5]. Similar behavior isobserved for Co0.89Cr1.73Se4 depicted in Fig. 5. The tem-perature dependence of magnetic susceptibility, χσ(T ),Fig. 2. The ZFC dc mass susceptibility χσ vs. temper-ature T for polycrystalline Cu0.93Co0.15Cr1.64Se4 andCu0.75Co0.25Cr2Se4 ferromagnetic spinels recorded atHdc = 100 Oe.Fig. 3. The ligand–field split components showing thesymmetry-lowered phase transitions in CuxCoyCrzSe4for high Co content (y ≥ 0.8).reveals antiferromagnetic-like behaviour with TN = 6 Kand without a Curie–Weiss behavior in the temperaturerange 4.2–400 K. Above TN the χσ(T ) curve shows min-imum at T1 = 118 K and maximum at T2 = 286 K.These results seem to be connected with thermally in-duced LS–HS transitions in the CuxCoyCrzSe4 systemfor y ≥ 0.8. For the Co2+ ion with 3d7 electronic config-urations the multiplet width should be comparable to kTbecause the temperature dependence of magnetic suscep-tibility, described by the Curie–Weiss law is not obeyedand the magnetic moment is strongly lowered. Only aslight decrease in magnetic moment could be related tothe magnetic moment of Co, which would be perpendic-ular to the magnetic moment of Cr. It takes into theeffect that the longitudinal component of the moment ofthe Co ions becomes zero and only the Cr moment re-mains visible in the saturation state, as it was found forMnCr2S4 [9] and Mn[Cr0.5Ga1.5]S4 [10].At T2 = 255 K for Cu0.14Co0.87Cr1.83Se4 and atT2 = 286 K for Co0.89Cr1.73Se4 thermal energy kT2 =Spin Crossover in CuxCoyCrzSe4 Semiconductors 713Fig. 4. The ZFC dc mass susceptibility χσ and 1/χσvs. temperature T for Cu0.14Co0.87Cr1.83Se4 recordedat Hdc = 800 Oe. TC is the Curie temperature and T1and T2 are temperatures characteristic for spin crossovertransition.Fig. 5. The ZFC dc mass susceptibility χσ and 1/χσvs. temperature T for Co0.89Cr1.73Se4 recorded atHdc = 800 Oe. TN is the Néel temperature and T1and T2 are temperatures characteristic for spin crossovertransition.0.0213 eV and 0.0238 eV, respectively. These energies areover 20 times smaller in comparison with the tetrahedralorbital splitting of Co2+ ion, ∆t = 0.5514 eV. A loweringsymmetry, displayed in Fig. 3, from cubic (4¯3m) to mo-noclinic (2/m) [11] makes easier the spin crossover tran-sitions as the orbital splitting is lowered giving the value|∆ − P | comparable to kT . We suggest that the broadhumps on the χσ(T ) curves above the ordering temper-ature may correspond to the 4T ↔ 2E spin crossover incobalt(II).4. ConclusionsWe have observed an anomalous behaviour of the tem-perature dependence of magnetic susceptibility and mag-netization of the CuxCoyCrzSe4 compounds for highcobalt content (y ≥ 0.8) in comparison with low cobaltone (y ≤ 0.25). The high content of the Co-ions weakensthe FM long-range interactions. Therefore, with increas-ing Co content a transition from ferromagnetic order viaferrimagnetic one to antiferromagnetic-like behaviour isobserved. This transition is accompanied with a loweringsymmetry from cubic (4¯3m) to monoclinic (2/m) and forthe latter the spin crossover phenomenon occurs. Thebroad humps at 255 K for Cu0.14Co0.87Cr1.83Se4 and at286 K for Co0.89Cr1.73Se4 on the χσ(T ) curves above theordering temperature are characteristic for the HS–LStransitions. The nature of these transitions seems to beconnected with the spin–orbit coupling giving the multi-plet width comparable to thermal energy.AcknowledgmentsThis work is partly founded from science grant No. NN204 289134.References[1] E. Maciążek, A. Molak, T. Goryczka, J. Alloys Comp.441, 222 (2007).[2] V. Svitlyk, Y. Mozharrivskyj, Inorg. Chem. 48, 5296(2009).[3] B.L. Morris, P. Russo, A. Wold, J. Phys. Chem. Solids31, 635 (1970).[4] T. Groń, E. Maciążek, J. Heimann, J. Kusz,I. Okońska-Kozłowska, K. Bärner, and Ch. Kleeberg,Physica B 254, 84 (1998).[5] P. Gütlich, Spin Crossover in Iron(II)–Complexes inStructure & Bonding, Vol. 44, Springer, Berlin 1981.[6] R. Sitko, B. Zawisza, E. Malicka, Spectrochim. Acta B64, 436 (2009).[7] V.I. Nizhankovskii, L.B. Lugansky, Meas. Sci. Tech-nol. 18, 1533 (2007).[8] O. Yamashita, Y. Yamaguchi, I. Nakatani, H. Watan-abe, K. Masumoto, J. Phys. Soc. Jpn. 46, 1145(1979).[9] V. Tsurkan, M. Mücksch, V. Fritsch, J. Hemberger,M. Klemm, S. Klimm, S. Körner, H.-A. Krug, D. vonNidda, E.-W. Samusi, A. Scheidt, S. Loidl, R. Horn,Tidecks, Phys. Rev. B 68, 134434 (2003).[10] T. Groń, J. Krok-Kowalski, A.W. Pacyna, T. Myd-larz, J. Phys. Chem. Solids 70, 900 (2009).[11] C.F. van Bruggen, Ann. Chim. Fr. 7, 171 (1982).

Spin crossover in CuxCoyCrzSe4 semiconductors

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Spin crossover in CuxCoyCrzSe4 semiconductors

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